When metal particles are nano-sized, they are capable of exhibiting physical, chemical and optical properties which are not possessed by conventional bulk metal. In particular, concerning their application as catalysts, metal nanoparticles have a considerably large reactive surface area and surface specificity and thus manifest very high reactive activity which is not shown in general catalysts. However, because nanocatalysts may aggregate upon synthesis thereof and are not easy to prepare in a uniform size, they are mainly used in a state of being immobilized on activated carbon, inorganic oxide, polymer, zeolite, etc. Also, these catalysts have low reusability and are unsuitable for actual reactions because they have high catalytic reaction temperature or should be used in a large amount, and furthermore need a ligand such as ascorbic sodium or amine.
Meanwhile, a transition metal catalyst such as a palladium catalyst is utilized in various organic reactions (Negishi, E.-I. In Handbook of Organopalladium Chemistry for Organic Synthesis; Negishi, E., Ed.; John Wiley & Sons: New York, 2002). In particular, this catalyst is very effective for the reduction of alkenes or alkynes, oxidation of alcohol, olefinization of allyl halides or a carbon-carbon coupling reaction such as cross-coupling (Denmark, S. E.; Sweis, R. F. Acc. Chem. Res. 2002, 35, 835).
A homogenous catalyst which is typically used is difficult to separate or reuse, and incurs problems related to industrial applications thereof. In order to solve these problems, a heterogeneous catalyst is developing. Most heterogeneous catalysts are provided in the form in which palladium is immobilized on an inorganic support or an organic polymer (Djakovitch, L. et al., J. Am. Chem. Soc. 2001, 123, 5990). Furthermore, there are reported reusable heterogeneous catalysts including palladium nanoparticles, colloidal palladium, palladium entrapped in a polymer and so on (Kim, S.-W. et al., J. Am. Chem. Soc. 2002, 124, 7642; Roucoux, A. et al., Chem. Rev. 2002, 102, 3757; and Akiyama, R. et al., J. Am. Chem. Soc. 2003, 125, 3412). However, the aforementioned catalysts are problematic in terms of low reactivity, decomposition of the catalyst, leaching of the metal and complicated synthesis procedures.
Korean Unexamined Patent Publication No. 10-2006-0076419 discloses a method of preparing a heterogeneous transition metal catalyst including mixing a transition metal complex, a carrier precursor and a ligand for capturing metal thus preparing a mixture, and adding water to the mixture to induce a sol-gel reaction.
However, the palladium-based catalyst is undesirably expensive. Although the preparation of a catalyst using inexpensive metal continues, the catalyst thus prepared is problematic in that its activity is insufficient.